Include json_inttypes.h in json_object.h since we use types like int32_t in the API and some systems need that header to compile.

As part of this create a public json_config.h with a custom define to
decide whether to include inttypes.h to avoid conflicting with other
projects config.h header.

json_config.h.in

@@ -0,0 +1,3 @@
+
+/* Define to 1 if you have the <inttypes.h> header file. */
+#undef JSON_C_HAVE_INTTYPES_H